Electro-optical scanners are widely used for reading bar codes, including one dimensional and two dimensional bar codes. A scanner typically includes a scanning module which: generates a scanning beam; repetitively directs and scans the beam across a target object, such as a bar code; receives reflected light from the target object; and digitizes and decodes the reflected light to decode the information encoded in the bar code. The scanning module is supported in a housing of the hand held portable scanner which also supports a power supply and other electronics of the scanner.
The scanning module scanning beam (typically a laser beam emitted by a laser diode) is directed at an oscillating scanning mirror. The oscillating scanning mirror, in turn, directs the beam outwardly through an exit window of the scanner. The oscillation of the oscillating reflector causes the beam to oscillate across a target object such as a bar code to be read. Essentially, the beam generates a beam spot that repetitively moves across or scans the bar code.
The light-colored or space elements of the bar code reflect the laser beam illumination and the dark or black bar elements of the bar code absorb the laser beam. Reflected light from the target bar code is received by a second reflective surface, such as a collection mirror, and directed toward photodetector circuitry, such as a photodiode. The pattern of reflected light, as received by the photodiode of the scanning module, is a representation of the pattern of the bar code. That is, a sequence of time when the photodiode is receiving reflected light represents the laser beam spot moving across a space of the bar code, while a sequence of time when the photodiode is not receiving reflected light represents the laser moving across a dark bar. Since the scanning speed or velocity of the reciprocating movement of the laser is known, the elapsed time of the photodiode receiving reflected light can be converted into a width of a bar code element corresponding to a space, while the elapsed time of the photodiode not receiving reflected light can be converted into a width of a bar code element corresponding to a bar.
The photodiode is part of photodetector circuitry which converts the reflected light into an analog signal. The scanning module includes an A/D converter or digitizer to digitize the analog signal generated by the photodiode. The digitizer outputs a digital bar code pattern (DPB) signal representative of the bar code pattern. A decoder of the scanning module inputs the DPB signal and decodes the bar code. The decoded bar code typically includes payload information about the product that the bar code is affixed to. Upon successful decoding of the scanned bar code, the scanner may provide an audio and/or visual signal to an operator of the scanner to indicate a successful read and decode of the bar code. The scanner typically includes a display to display payload information to the operator and a memory to store information decoded from the bar code.
One type of electro-optical scanner, referred to as a retro-reflective scanner, employs retro-reflective light collection. In a retro-reflective scanner, the scanning module includes a mirror that both: 1) directs the laser beam toward the target bar code or another mirror; and 2) receives reflected light from target bar code and directs it toward the photosensor circuitry. An example of such a retro-reflective scanner is disclosed in U.S. Pat. No. 6,360,949 to Shepard et al., assigned to the assignee of the present invention. The '949 patent is incorporated herein in its entirety by reference.
As size considerations are extremely important in portable, hand held scanners, what is needed is a retro-reflective electro-optical scanner that provides for reduced size of scanning module components, specifically the light collection mirror, while maintaining the ability to read wide bar codes.